1. Field of the Invention
The present invention relates to a process for the removal of metal poisons from a hydrocarbon conversion catalyst which has been contaminated with one or more poisoning metals by use in a high temperature catalytic conversion of hydrocarbon feedstocks to more valuable, lower boiling products. More particularly, the present invention relates to an improved process of reducing the vanadium content of such catalysts. The invention may be used as part of an overall metals-removal process employing a plurality of processing steps to remove a significant amount of one or more of nickel, vanadium and iron contained in the poisoned catalyst.
2. Description of the Background
Catalytically promoted methods for the chemical conversion of hydrocarbons include cracking, hydrocracking, reforming, hydrodenitrogenation, hydrodesulfurization, etc. Such reactions generally are performed at elevated temperatures, for example about 300.degree. to 1200.degree. F., more often 600.degree. to 1000.degree. F. Feedstocks to these processes comprise normally liquid and solid hydrocarbons which, at the temperature of the conversion reaction, are generally in the fluid, i.e., liquid or vapor state, and the products of the conversion usually are more valuable, lower boiling materials.
In particular, cracking of hydrocarbon feedstocks to produce hydrocarbons of preferred octane rating boiling in the gasoline range is widely practiced and uses a variety of solid inorganic oxide catalysts to give end products of fairly uniform composition. Cracking is ordinarily effected to produce gasoline as the most valuable product and is generally conducted at temperatures of about 750.degree. to 1100.degree. F., preferably about 850.degree. to 950.degree. F., at pressures up to about 2000.degree. psig. and without substantial addition of free hydrogen to the system. In cracking, the feedstock is usually a petroleum hydrocarbon fraction such as straight run or recycle gas oils or other normally liquid hydrocarbons boiling above the gasoline range. Recently, low severity cracking conditions have been employed for heavily contaminated feedstocks such as crude or reduce crude where the conversion is not made directly to the most valuable, lower boiling products, i.e. gasoline boiling range products, but to intermediate type hydrocarbon conversion products which may be later refined to the more desirable, lower boiling, gasoline or fuel oil fractions. High severity cracking has also been practiced for the conversion of such feedstocks to light, normally gaseous hydrocarbons, such as ethane, propane or butane.
The present invention relates to the improvement of catalyst performance in hydrocarbon conversion where metal poisoning occurs. Although referred to as "metals", these catalystic contaminants may be present in the hydrocarbon feed in the form of free metals or relatively non-volatile metal compounds. It is, therefore, to be understood that the term "metal" as used herein refers to either form. Various petroleum stocks have been known to contain at least traces of many metals. For example, Middle Eastern crudes contain relatively high amounts of several metal components, while Venezuelan crudes are noteworthy for their vanadium content and are relatively low in other contaminating metals such as nickel. In addition to metals naturally present in petroleum stocks, including some iron, petroleum stocks also have a tendency to pick up tramp iron from transportation, storage and processing equipment. Most of these metals, when present in a stock, deposit in a relatively non-volatile form on the catalyst during the conversion processes so that regeneration of the catalyst to remove deposited coke does not also remove these contaminants. With the increased importance of gasoline in the world today and the shortages of crude oils and increased prices, it is becoming more and more important to process any type or portion of the crude, including the highly metal contaminated crudes to more valuable products.
Typical crudes which are contaminated with metals and some average amounts of metal are: North Slope, 11 ppm nickel, 33 ppm vanadium; Lagomedio (Venezuelan), 12 ppm nickel, 116 ppm vanadium; light Iranian, 16 ppm nickel, 44 ppm vanadium; heavy Iranian, 30 ppm nickel, 22 pp[m vanadium. In general, a crude oil can contain from about 5 to 500 ppm nickel and from about 5 to 1500 ppm vanadium. Moreover, since the metals tend to remain behind during processing, the bottoms of typical feeds will have an amount of metals two, three, four times or more than the original crude. For example, reduced crude or residual stocks can have vanadium levels as high as 1000-2000 ppm. Typical residual stocks and their vanadium level include: Sag River atmospheric residuum, 48 ppm vanadium; heavy Iranian atmospheric residuum, 289 ppm vanadium; Canadian tar sand bitumen, 299 ppm vanadium; Ti Juana Vacuum residuum, 570 ppm vanadium; Bachaquero Vacuum residuum, 754 ppm vanadium; and Orinoco Heavy Crude, 1200 ppm vanadium. The higher the metal level in the feed, the more quickly a given catalyst will be poisoned and consequently the more often or more effective the demetallization of that catalyst must be.
Of the various metals which are to be found in representative hydrocarbon feedstocks some, like the alkali metals, only deactivate the catalyst without changing the product distribution; therefore, they might be considered true poisons. Others such as iron, nickel, vanadium and copper markedly alter the selectivity and activity of cracking reactions if allowed to accumulate on the catalyst and, since they affect process performance, are also referred to as "poisons". A poisoned catalyst with these metals generally produces a higher yield of coke and hydrogen at the expense of desired products, such as gasoline and butanes. For instance, U.S. Pat. No. 3,147,228 reports that it has been shown that the yield of butanes, butylenes and gasoline, based on converting 60 volume percent of cracking feed to lighter materials and coke dropped from 58.5 to 49.6 volume percent when the amount of nickel on the catalyst increased from 55 ppm to 645 ppm and the amount of vanadium increased from 145 ppm to 1480 ppm in a fluid catalytic cracking of a feedstock containing some metal contaminated stocks. Since many cracking units are limited by coke burning or gas handling facilities, increased coke or gas yields require a reduction in conversion of throughput to stay within the unit capacity.
An alternative to letting catalyst metals level increase and activity and desired selectivity decrease is to diminish the overall metal content on the catalyst by raising catalyst replacement rates. Either approach, letting metals level increase, or increasing catalyst replacement rates, must be balanced against product value and operating costs to determine the most economic way to operating. The optimum metal level at which to operate any cracking unit will be a function of many factors including feedstock metal content, type and cost of catalyst, overall refinery balance, etc., and can be determined by a comprehensive study of the refinery's operations. With the high cost of both catalyst and the hydrocarbon feedstock today, it is increasingly disadvantageous to discard catalyst or convert hydrocarbon feedstocks to coke or gas.
Many patents have issued discussing various approaches to removing metals from hydrocarbon conversion catalysts and then returning the catalyst to hydrocarbon conversion service. See, for example, U.S. Pat. Nos. 3,150,103; 3,150,104; 3,122,510; 3,173,882; 3,147,228; 3,219,586; 3,182,025; 3,252,918; 4,101,444; 4,163,709; 4,163,710; 4,243,550; and 4,686,197.
As disclosed in U.S. Pat. Nos. 4,686,197, and 4,243,550, both of which are incorporated herein by reference, a typical treatment of a metal poisoned catalyst includes regeneration in which portions of the catalyst are periodically contacted with free oxygen containing gas to removal at least a portion of the carbonaceous deposits, sulfiding in which the regenerated catalyst is contacted with sulfiding agents, e.g. H.sub.2 S, to convert the metals into metal-sulphur compounds to produce a sulfided catalyst, and chlorination in which the sulfided catalyst is contacted with a chlorine containing compound to convert the metal poisons to metal chlorides which can be removed by volatilization and/or aqueous washing. The catalysts can also be subjected to other processes such as oxidation, reductive washes, oxidative washes, etc., all of which are aimed at effecting some removal of the metal poisons.
Sulfiding of the poisoned catalysts is known to be highly advantageous for nickel removal but less so for the removal of vanadium. For example, it is known that greater than 80 percent of the nickel can be removed using conventional, prior are demetallization processes but the removal of vanadium is significantly less.